Exploration for Platinum-Group Minerals in Till: a New Approach to the Recovery, Counting, Mineral Identiﬁcation and Chemical Characterization

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Article Exploration for Platinum-Group Minerals in Till: A New Approach to the Recovery, Counting, Mineral Identiﬁcation and Chemical Characterization

Sheida Makvandi *, Philippe Pagé, Jonathan Tremblay and Réjean Girard

IOS Services Géoscientiﬁques Inc., 1319 Boulevard Saint-Paul, Chicoutimi, QC G7J 3Y2, Canada; [email protected] (P.P.); [email protected] (J.T.); [email protected] (R.G.) * Correspondence: [email protected]; Tel.: +1-418-698-4498

Abstract: The discovery of new mineral deposits contributes to the sustainable mineral industrial development, which is essential to satisfy global resource demands. The exploration for new mineral resources is challenging in Canada since its vast lands are mostly covered by a thick layer of Quaternary sediments that obscure bedrock geology. In the course of the recent decades, indicator minerals recovered from till heavy mineral concentrates have been effectively used to prospect for a broad range of mineral deposits including diamond, gold, and base metals. However, these methods traditionally focus on (visual) investigation of the 0.25–2.0 mm grain-size fraction of unconsolidated sediments, whilst our observations emphasize on higher abundance, or sometimes unique occurrence of precious metal (Au, Ag, and platinum-group elements) minerals in the finer-grained fractions (<0.25 mm). This study aims to present the advantages of applying a mineral detection routine initially developed for gold grains counting and characterization, to platinum-group minerals in Citation: Makvandi, S.; Pagé, P.; <50 µm till heavy mineral concentrates. This technique, which uses an automated scanning electron Tremblay, J.; Girard, R. Exploration microscopy (SEM) equipped with an energy dispersive spectrometer, can provide quantitative min- for Platinum-Group Minerals in Till: A New Approach to the Recovery, eralogical and semi-quantitative chemical data of heavy minerals of interest, simultaneously. This Counting, Mineral Identification and work presents the mineralogical and chemical characteristics, the grain size distribution, and the Chemical Characterization. Minerals surface textures of 2664 discrete platinum-group mineral grains recovered from the processing of 2021, 11, 264. https://doi.org/ 5194 glacial sediment samples collected from different zones in the Canadian Shield (mostly Quebec 10.3390/min11030264 and Ontario provinces). Fifty-eight different platinum-group mineral species have been identified to

date, among which sperrylite (PtAs2) is by far the most abundant (n = 1488; 55.86%). Textural and Academic Editors: Derek H. mineral-chemical data suggest that detrital platinum-group minerals in the studied samples have C. Wilton and Gary Thompson been derived, at least in part, from Au-rich ore systems.

Received: 4 February 2021 Keywords: platinum-group elements; platinum-group mineral; glacial sediments; indicator minerals; Accepted: 27 February 2021 heavy mineral concentrate; automated SEM-EDS mineral analysis Published: 4 March 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional afﬁl- iations. Despite historically high mineral exploration spending over the past decades, there has been no corresponding increase in new discoveries [1]. The industry is facing a challenge in regard of resource renewal, and new exploration methods are needed, particularly for targeting ore deposits buried under glacial sediment and areas of transported cover that does not respond properly to geophysics, such as in vast areas of the subarctic zone Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. (e.g., Alaska, Canada, Scandinavia, Siberia) [2]. In such environment, the glacial sediments, This article is an open access article although perceived as a hindrance to exploration, led to development of methods capable distributed under the terms and of detecting the signal from the mineral deposits as they are eroded and dispersed in the conditions of the Creative Commons sediments. Till geochemistry and indicator mineral methods have therefore been instru- Attribution (CC BY) license (https:// mental in discovering underlying mineral resources [3]. Dominantly used for gold [4] creativecommons.org/licenses/by/ and diamond exploration [5–7], the technique of using the dispersion of minerals consid- 4.0/).

Minerals 2021, 11, 264. https://doi.org/10.3390/min11030264 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 264 2 of 28

ered as footprints of metallogenic environment has recently expanded interest to other commodities including Ni-Cu-platinum-group element deposits [8–11]. Platinum, Pd, Rh, Ir, Ru, and Os, known as platinum-group elements (PGE), are used in a wide variety of industrial applications (vehicles catalysts, fertilizer synthesis, etc.) that are considered as strategic and critical worldwide [12]. Still, approximately 90 percent of PGE originates from South Africa and Russia and they are barely mined out in other industrialized countries [13]. This issue mostly relates to the difficulty of prospecting for this group of metals, notably due to their low abundance, mineralogical complexities, and lack of geophysical or geochemical signatures outside of nickel sulfide occurrences. Hence, the current work aims at presenting the results obtained over the course of the last five years of processing glacial sediment samples using an automated mineral sorting method capable to detect, analyze, and classify platinum-group minerals (PGM) from glacial sediments, and present the usefulness of this new approach for the exploration of PGE-bearing deposits.

1.1. Platinum-Group Elements (PGE) and Their Distribution Platinum group elements have similar physical and chemical properties and occur closely related in nature. These metals are of the rarest elements occurring in the bulk Earth crust, and due to their highly siderophile nature, are mainly concentrated in the dense metallic core [14]. The PGE belong to Group VIII transition metals in the periodic table of elements with Fe, Co, and Ni. Thus, in natural systems they have the ability to be engaged in both metal-metal and covalent bonding to form solid solution with one another, or compounds with siderophiles (e.g., Fe), chalcogenides (e.g., S, Se, Te), or semi-metals (e.g., As, Sb, Te) [8,15]. In nature, these metals form PGE-dominated complexes and alloys, which are known as the platinum-group minerals (PGM), which constitute a very diverse group of minerals (>140 named discrete PGMs) [16]. In magmatic settings, in presence of sulfur or semi-metals (such as As) and due to instability of metallic Fe, the PGE behave as highly chalcophile instead of siderophile elements [16]. Thus, the PGE have high partition coefficients for sulfides in sulfur-saturated magmatic systems [17,18]. Hence, about 99% of the global PGE resources are hosted in magmatic sulfide deposits [19]. Thus, although geochemical characteristics of the PGE allow them to be involved in a great variety of geological processes, our insight into their distribution, and PGMs formation and diversity have mainly been developed by the study of magmatic systems, mostly large layered mafic-ultramafic intrusions (e.g., Bushveld, Stillwater, and Great Dyke complexes). In these, major PGE ore deposits are found as reef- type deposits [17,20], magmatic breccia type such as the Lac des Iles palladium deposit [20], and conduit-type deposits such as those in the Noril’sk-Talnakh area (Russia), which formed as a result of circulation and eruption of vast volume of mafic magma that formed the Siberian Trap Large Igneous Province [21]. Other magmatic deposit types having their importance for the study of the PGMs diversity and composition come from Uralian- Alaskan-type complexes and ophiolite complexes ([22] and references therein). In contrast to the PGMs associated with magmatic systems, there is a poor understanding about the PGEs’ distribution and mobility during hydrothermal activities and how PGMs may physiochemically modify under low temperature conditions and/or in the surface secondary environment. This is important because the PGMs concentration in placer deposits principally originates from low temperature degradation of large mafic layered intrusions, ophiolitic and/or Uralian-Alaskan-type complexes [23]. Different studies suggest that PGMs composed of native metals and alloys do not typically undergo phase transitions within temperature and/or pressure ranges of geological significance, and they also tend to be relatively resistant to geochemical processes during which lithophile elements can be fractionated and modified in silicate and non-silicate rocks [24–26]. Nevertheless, Hanley [27] discussed the alteration of PGM assemblages precipitated from high temperature liquids at lower temperatures during metamorphism, hydrothermal processes, and/or surficial weathering. Minerals 2021, 11, 264 3 of 28

1.2. Exploration for Platinum-Group Minerals For a long time, placer deposits were the only PGM sources, but currently their contribution to the global resources is less than 4% [28]. The discovery of some primary PGE deposits such as the Merensky Reef of the Bushveld Complex (South Africa), the JM Reef of the Stillwater Complex (MT, USA), the Main Sulfide Zone of the Great Dyke (Zimbabwe), the Noril’sk-Talnakh deposits (Russia), the Sudbury Igneous Complex (Canada), and the Lac des Iles Complex (Canada) has decreased the role of PGM placers as single PGE sources. However, the study of PGM assemblages in dispersion halos remains topical as a prospecting tool for both primary and placer PGE deposits [10]. Nowadays, indicator mineral methods are significantly used exploration tools for detecting a variety of ore deposit types including diamond, gold, Ni-Cu, PGE, U, porphyry Cu, massive sulfide, and tungsten deposits [3,29–39]. A great advantage of indicator mineral methods is their extremely low detection limits that enable tracing the dispersion of eroded mineral deposits in surficial glacial sediments, in which minerals contain a wealth of information in regard of the metallogenic environment they originate from. Platinum-group minerals seem to be the best indicator for tracing their own deposits including magmatic deposits (e.g., Ni-Cu) and the less common hydrothermal and sedimentary deposit types ([13] and references therein). The abundance of indicator minerals such as Au and PGM in sediments depends on their contents in the primary source rocks/deposits, the degree of weathering of these sources, mechanisms of transport and dispersion of indicator minerals after liberation from host rocks/deposits, and their ability to resist against mechanical and/or chemical modifications (e.g., fluvial, glacial, fluvioglacial, aeolian) [8,40]. This also controls the efficiency of techniques used to concentrate and sort these minerals. These factors must be taken into account in every sampling plan [40].

1.3. Conventionnal Analytical Methods: MLA and QEMSCAN Heavy mineral concentrates with the grain sizes <50 µm commonly contain fine- grained indicator minerals that either formed originally into minute crystals or were comminuted and abraded during transport. Most gold and PGM grains are silt-size and occur in the < 50 µm heavy mineral fractions [8,41,42]. Given the small nature of the grains, their diversity, and their typically metallic grey colors that confuse them with numerous other mineral species, preparing representative samples and/or samples suitable for optical examination and geochemical characterization of Au and PGMs using conventional analytical methods is neither time- nor cost-effective. From developments in the 1970s [43], the use of automated scanning electron microscopy (SEM) such as MLA (mineral liberation analysis) and QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscope) has enabled circumventing the difficulties associated with their visual sorting from the <250 µm fraction of heavy mineral concentrates (HMC) from samples. The advantages of applying these automated SEM- based techniques include: (1) efficiency and rapidity—energy dispersive (ED) spectra of 200,000 particles (median particle size of 30 µm) can be collected within 6 h [44]; (2) the detection of grains of low abundances (one grain compared to initial samples is equivalent to ppt level, required to detect such low abundance metals; (3) the possibility of identifying and using new indicator minerals not previously recognized; (4) data collection on certain physical properties of minerals such as grain size distribution, mineral liberation, and mineral association, which can contribute to provenance discrimination [45–47]. In spite of the aforementioned advantages, neither MLA nor QEMSCAN can provide quantitative chemical analysis, both using EDS spectrum matching and analytical proxy stored in a library. Thus, these techniques are fairly rigid in regard of which minerals they detect, and thus face challenges: (1) in identification of unexpected mineral species that their corresponding energy dispersive X-ray spectra do not already exist in the mineral reference library; (2) as typical formula of each mineral is used, compositional variation in minerals (e.g., Au in potarite, Ag and/or Pd enrichment in Au particles), which reflects characteristics of their source, may not be distinguished. Accordingly, MLA and QEM- Minerals 2021, 11, 264 4 of 28

SCAN are limited for the characterization of PGMs; for instance, because these can form complex phase series with elemental substitutions (cooperite, braggite, and vysotskite), solid-solution alloys (e.g., Ir-rich osmium and Os-rich iridium) that cannot be differentiated by these techniques. As for gold, the characterization method must be able to estimate trace elements abundances (Te, Hg, Ag, Pd, Cu) considering their implications in provenance studies [48]. To overcome the given restrictions when dealing with HMC > 250 µm, comple- mentary studies commonly consider such mineral identification using optical microscopy, and chemical and crystallographic analyses such as EPMA (electron probe micro-analyzer), LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry), and EBSD (electron backscatter diffraction). However, as mentioned earlier, these methods are not properly applicable to very fine mineral fractions because, even if these minerals can be located in the heavy mineral concentrate, they can hardly be manipulated and the grain size or inclusion- or alteration-free surface of the majority of these minerals would be smaller than the beam size used in LA-ICP-MS. Moreover, as MLA or QEMSCAN are not capable to scan more than about 40,000 grains per hour, analyzing a grain mount of <50 µm HMC may take numerous hours. Thus, such studies are commonly costly and are not affordable for most mineral exploration companies. Another limitation with MLA or QEMSCAN applied to fine-grained fraction of HMC is that samples shall be mounted in epoxy and polished. Manufacturing and polishing a monolayer mineral mount is difficult, meaning that an excess of material is required to obtain a volume that can be cut and polished. It also means that the surface textures of the grains are removed and their outline/shape is affected. Many studies have shown that the surface textures of indicator minerals are diagnostic of transport media and physicochemical conditions of secondary environment in which detrital grains are deposited ([49] and references therein). Moreover, the shape and surface texture of grains, whether they are affected by erosion marks, is historically used to estimate the distance that the grains travelled from their source rocks [50]. Finally, the most underrated difficulty in using PGM as indicator mineral remains their intrinsic smallness. Platinum-group minerals rarely exceed 100 µm in diameter and, therefore, may not be efficiently recovered by conventional heavy mineral concentration techniques. Their size is more comparable to gold grains, the concentration and sorting of which require different techniques than those usually applied to indicator minerals. At such size, they are not efficiently recovered by gravimetric methods, either at shaking table or heavy liquids. Since the quantity of material that can be scanned by MLA or QEM- SCAN is limited to a fraction of a gram (especially in precious metal minerals exploration program), more sophisticated concentration techniques are needed to achieve statistical representativeness [50]. In this regard, the current study circumvented these difficulties by introducing an approach based on a recent development on gold grain recovery (ARTGoldTM or Ad- vanced Recovery Technique for gold grains) complemented with automated SEM-based technology. These developments overcome the aforementioned restrictions in recovery and characterization of indicator minerals, are fully applicable for the platinum-group minerals, and can be implemented as routine procedure in the course of regional gold exploration surveys. The current study investigates a database of 2664 PGM grains recovered from 5194 glacial sediments samples from 24 different surveys. The exact sampling locations of these projects are not disclosed to reserve the IOS Services Géoscientifiques (IOS-SG) clients’ rights and no issues relating to mineral exploration within these projects are to be discussed. As shown in Figure1, most of the surveys are from various parts of the Superior Province, a major Archean craton of the Canadian Shield, dominantly in the Abitibi and James Bay areas. Still, two surveys are from the lower Proterozoic Cape Smith belt New-Québec Orogen, Nunavik region, two others are from the middle Proterozoic Grenville Province, and one is from the upper Paleozoic of the Appalachian (Figure1). It must be noted that all these samples have originally been collected for gold exploration or regional assessment, and none was specifically for PGM sources. Minerals 2021, 11, x FOR PEER REVIEW 5 of 28

Abitibi and James Bay areas. Still, two surveys are from the lower Proterozoic Cape Smith belt New-Québec Orogen, Nunavik region, two others are from the middle Proterozoic

Minerals 2021, 11, 264 Grenville Province, and one is from the upper Paleozoic of the Appalachian (Figure5 of 28 1). It must be noted that all these samples have originally been collected for gold exploration or regional assessment, and none was specifically for PGM sources.

FigureFigure 1. (a) Map 1. (a )of Map geological of geological provinces provinces of Canada of Canada (modified (modiﬁed from fromWheeler Wheeler et al. et [51]). al. [ 51(b]).) Location (b) Location map mapof till of sampling till areas. sampling areas.

2. Materials2. Materials and and Methods Methods TheThe procedure procedure used used for for the the current studystudy has has been been developed developed and and implemented implemented in in the theIOS-SG IOS-SG installations installations originallyoriginally to to commercially commercially process process glacial glacial sediments sediments for their goldfor their goldgrain grain contents. contents. The The procedure procedure has been has developed been developed to automate to automate the gold grain the gold counting grain and count- systematize their concentration. At the onset of its use, the unexpected frequent occurrence ing and systematize their concentration. At the onset of its use, the unexpected frequent of PGM in samples has been noted, and data about these were systematically collected. occurrenceThe large of datasets PGM in generated samples afterwards has been isnoted, hereinvestigated and data about for (indicator) these were mineral systematically (and collected.surﬁcial The geochemical) large datasets exploration, generated geo-metallurgical afterwards is and/or here investigated geo-environmental for (indicator) purposes. mineral (and surficial geochemical) exploration, geo-metallurgical and/or geo-environmental purposes.2.1. Sample Preparation Collecting 10 to 20 kg of sandy sediment and 20 to 40 kg of clay-rich sediment is usually 2.1.recommended Sample Preparation for Au and PGE heavy mineral surveys/exploration programs [8,52,53]. However, according to industry practice, glacial sediment samples submitted for gold grain assessments are typically of 10 kg or less. Upon reception at the laboratory, samples

Minerals 2021, 11, 264 6 of 28

are registered and queued to be processed based on the ARTGoldTM protocol as follows. (1) Samples are soaked with a wetting agent and sieved in a mechanized vibrating screen stack to remove coarser (>1 mm) particles. Fine particles are decanted in a settling tank. (2) Gravimetric separation of wet sieved <1000 µm fraction proceeds with an in-house vibrating micro-corrugated sluice, or ﬂuidized bed. A super-concentrate of 100 to 200 mg is typically obtained from 10 to 20 kg till sample using a single pass. (3) To assess the recoveries (quality control vs. quality assurance: QA/QC), tails of the ﬂuidized bed are stored and reprocessed for about 10% of the sample population. (4) Magnetite and other ferromagnetic minerals are removed from the dried super-concentrate with a hand magnet. The super-concentrates received in the micro-analysis laboratory are sieved with a dis- posable 50 µm woven polyester mesh. The material >50 µm is sent for visual examination under a research grade stereomicroscope for standard visual sorting (“picking”), whereas the material <50 µm is dusted on a 4 × 4 cm double sided carbon tape to form a monolayer of heavy mineral grains for automated scanning electron microscope routine.

2.2. Analytical Method ARTGoldTM is originally an automated gold grain counting routine implemented on a SEM and based on the AZtec Feature platform (Oxford Instruments, OI, Abingdon, UK). Two instruments were used, a Zeiss EVO MA15 HD (Carl Zeiss AG, Oberkochen, 2 Germany) with a LaB6 emitting source equipped with an OI X-Max 150 mm energy dispersive spectrometer (EDS-SDD) and a Zeiss Sigma 300 VP FEG-SEM (Carl Zeiss AG, Oberkochen, Germany) coupled with the OI Ultim-Max 170 mm2 detector. The method relies on backscattered images, which are acquired using multi-segments backscattered electron detector to discriminate grain density. The EDS analyses, trigged on minerals with an apparent atomic density in excess of 5 g/cc, are semi-quantitative with the current configuration, as they are conducted with a short counting time, a high working distance (11–12 mm), are standardless (using factory calibration) and are normalized to 100%. Analyses are performed under variable pressure mode (40 Pa) and with an accelerating voltage of 20 kV. Using partial pressure helps in regard to draining the electron, the principle being that nitrogen in the chamber is ionized by the electron beam, creating a channel of plasma that conducts the electron. However, the main reason is to avoid excessive outgassing of the carbon-tape substratum that causes distortion of the surface. Specimen current is monitored daily on the Zeiss EVO as it varies overtime due to filament wearing. It is adjusted to ensure maximum output count rate with the EDS, with less than 30% dead time. The current on the Sigma 300 VP is stable overtime and the need of adjustment is limited. The AZtec 4.2 software (OI, Abingdon, UK) is used to acquire the BSE images, to detect particles, to acquire EDS spectra, and for their deconvolution into chemical analyses. Nitrogen, present in the chamber under variable pressure mode, causes the diffusion of the electron beam, which then interacts with the material adjacent to the targeted area over a distance estimated to 100 µm radius. Thus, some elements from the substrate, such as carbon and chlorine, have been excluded from the analyses and the signal on the gold or PGM grains is typically be contaminated by the adjacent mineral phases. The analyses are thus normalized to 100% for the range of elements of interest, excluding contaminants. A mosaic of 520 to 560 backscattered electron (BSE) images of 1.92 mm × 1.44 mm (1024 × 768 pixels for a resolution of 1.88 µm/pixel) are acquired per sample mounts to locate and then identify heavy mineral phases (e.g., gold, bismuthides and tellurides, PGMs, scheelite, galena, etc.) among more than one million grain of the 0–50 µm size fraction on a single 3.5 cm diameter circular mounts. Minerals with a density in excess of 5 g/cc are discriminated given their relatively high brightness correlating with their high average atomic number. The brightness threshold is set to exclude monazite, as this mineral is ubiquitous in the heavy mineral fraction and would hinder the speed of the acquisition process. Most minerals of interest have a density higher than monazite, and the effectiveness of recovery of minerals with a lower density on the fluidized bed was not investigated. Semi-quantitative chemical microanalyses Minerals 2021, 11, x FOR PEER REVIEW 7 of 28

Minerals with a density in excess of 5 g/cc are discriminated given their relatively high brightness correlating with their high average atomic number. The brightness threshold is set to exclude monazite, as this mineral is ubiquitous in the heavy mineral fraction and would hinder the speed of the acquisition process. Most minerals of interest have a density higher than monazite, and the effectiveness of recovery of minerals with a lower Minerals 2021, 11, 264 density on the fluidized bed was not investigated. Semi-quantitative chemical7 of 28 microanalyses (300,000 counts, or approximately 1–2 s) are automatically acquired in the center of every detected particle. Depending on how many very-dense grains exist in a super- concentrate that need an EDS analysis, scanning a 4 cm × 4 cm sample typically takes (300,000 counts, or approximately 1–2 s) are automatically acquired in the center of every detectedabout an particle. hour. Depending Minerals are on howclassified many very-densebased on their grains chemical exist in acomposition super-concentrate using a classi- thatfication need antree, EDS and analysis, those scanningcontaining a 4 cmAu,× Ag,4 cm or sample PGE are typically automatically takes about selected an hour. for further Mineralsanalyses. are In classified this step, based a high-resolution on their chemical BSE composition image of using 144 aμm classification × 108 μm tree,(1024 and × 768 pixels thosefor containinga resolution Au, of Ag, 0.14 or PGEμm/pixel) are automatically is acquired selected and a forpoint further analysis analyses. is performed In this with step,500,000 a high-resolution effective counts BSE image for ever of 144y preciousµm × 108 metalµm (1024 bearing× 768 grains. pixels for a resolution of 0.14 µm/pixel)Platinum-group is acquired minerals and a point are analysis then identifi is performeded based with on 500,000 their effective chemical counts compositions forand every using precious the K-means metal bearing algorithm grains. [54]. To this end, the chemistry of unknown minerals is Platinum-group minerals are then identified based on their chemical compositions compared to an evolving large in-house dataset built over time by compiling literature and using the K-means algorithm [54]. To this end, the chemistry of unknown minerals isdata compared of Au-, to Ag-, an evolving and PGE-bearing large in-house minerals. dataset builtThe classifier over time suggests by compiling the two literature most probable datamineral of Au-, names Ag-, and for PGE-bearing each mineral minerals. composition The classifier and report suggests the the classification two most probable accuracy. The mineralaccuracy names is generally for each mineral above composition 95%, except and in reportfew cases the classification where the accuracy.composition The does not accuracymatch the is generally existing aboveentries 95%, of the except training in few data casesset, where or where the composition chemical substitutions does not create matchcomplications the existing (e.g., entries Se-rich of the or training Cu-rich dataset, braggite), or where or near chemical the limits substitutions between createphases belong- complicationsing to mineral (e.g., series Se-rich (e.g., or Cu-richlaurite-erlichmani braggite), orte, near cooperite-braggite-vysotskite). the limits between phases belong- In these lat- ingter to cases, mineral a specialist series (e.g., clarifies laurite-erlichmanite, and fixes the cooperite-braggite-vysotskite).mineral identification using graph In these projections latter cases, a specialist clarifies and fixes the mineral identification using graph projections and calculations. and calculations. TM AtAt the the end end of every of every ARTGold ARTGoldTM project, project, a mosaic a mosaic of backscatter of backscatter electron (BSE)electron images (BSE) images ofof all all identified identified and and analyzed analyzed precious precious metal metal minerals minerals (PMM) (PMM) is created is tocreated provide to an provide an overviewoverview for for further further sediment sediment provenance provenance discrimination. discrimination. The mosaic The depictedmosaic depicted in Figure2 in Figure 2 illustratesillustrates 166 166 PMM PMM grains grains (excluding (excluding gold grains) gold foundgrains) in afound single in project a single sampled project in the sampled in Nunavikthe Nunavik region (Nunavik,region (Nunavik, QC, Canada). QC, TheCanada). PMMs The are sortedPMMs based are sorted on their based grain sizes.on their grain Itsizes. is worth It is mentioning worth mentioning that, in addition that, in to addition Au and PGMs, to Au Ag-bearingand PGMs, minerals Ag-bearing comprise minerals com- anotherprise another group of group PMM of detected PMM detected and analyzed and analyz by theed routine. by the Nativeroutine. Ag Native (Figure Ag 5a (Figure), 5a), acanthite (Ag2S; Figure 5b), hessite (Ag2Te; Figure 5c), and matildite (AgBiS2) are the acanthite (Ag2S; Figure 5b), hessite (Ag2Te; Figure 5c), and matildite (AgBiS2) are the com- common Ag-bearing minerals detected, though Ag is mostly concentrated in Au-Ag alloys inmon the studied Ag-bearing samples. minerals detected, though Ag is mostly concentrated in Au-Ag alloys in the studied samples.

FigureFigure 2. A2. mosaicA mosaic of 166 of precious166 precious metals metals minerals minerals (PMM) excluding(PMM) excluding Au is automatically Au is automatically generated gener- byated the routine.by the routine. These PMM These include PMM various include species various of platinum-groupspecies of platinum-group elements-bearing elements-bearing and Ag- and bearingAg-bearing minerals minerals found in found till samples in till collectedsamples in colle thected Nunavik in the area Nunavik (Quebec, area Canada). (Quebec, Canada).

Minerals 2021, 11, 264 8 of 28

3. Results 3.1. Distribution of Various Platinum-Group Mineral Species in Till A total of 2664 discrete PGM grains have been identified and analyzed in heavy mineral super-concentrates obtained from processing of till samples. The diversity and distribution of PGMs found in the studied samples are illustrated in Figures3 and4, and selected backscatter electron (BSE) images of relatively abundant PGE-bearing minerals are displayed in Figures5 and6. As a result, the recovered and analyzed PGMs (on average one PGM grain per two till samples) are composed of 58 different mineral species among which sperrylite (PtAs2) is by far the most abundant (Figure3). The sperrylite grains ( n = 1488) represent 55.86% of the total population. The Pt + Pd sulfides, including cooperite (PtS) and braggite [(Pt,Pd,Ni)S], are frequently observed (n = 240, 9.01%; and n = 227, 8.52%, respectively; Figure6a,b). In contrast, the Pd sulfide vysotskite is much less abundant as only 11 grains have been identified (0.4%). In Figure3, the category identified as Pt includes native platinum grains but also diverse Pt-dominated alloys. Iron is commonly alloyed with platinum, but since Fe is excluded from the analyses (due to secondary fluo- rescence), isoferroplatinum (Pt3Fe), tetraferroplatinum (PtFe), tulameenite (Pt2CuFe), and ferronickelplatinum (Pt2FeNi) could not be differentiated and, if present, they have been included in the Pt category. In addition, PGM species whose individual abundance is <0.5% (less than 13 grains) of the population are grouped as “Others” in Figure3. This group consists of a variety of rare PGM species (Table1) including taimyrite ( Figure6 f), cabriite (Figure6 g), paolovite (Figure6i), anduoite (Figure5d), atheneite, vysotskite, hollingworthite, tatyanaite, vincentite, mitrofanovite, hongshiite (PtCu; may be tulameenite but Fe is excluded from the analysis), palladium-dominated alloys, palladian weishanite (here referred to (Pd(±Pt)AuHg)), osarsite, stannopalladinite, naldretteite, niggliite, omeiite, ruarsite, zvyagintsevite, arsenopalladinite, chrisstanleyite, iridarsenite, palarstanide, palladseite, and rustenburgite. There are also some mineral species that have only been detected as single grains (equivalent of ppt level) including kotulskite, merenskyite, atokite, kingstonite, padmaite, palladobismutharsenide, plumbopalladinite, sudovikovite, telargpalite, and ungavaite. Backscatter electron images of PGMs in Figures5 and6 exemplify how PGMs in surficial sediment differ in grain sizes, shapes, and textures (e.g., free or interlocked minerals; euhedral, fractured, or with abraded edges). Based on the geochemical behavior of the PGE, many studies have shown that Os, Ir, and Ru (the iridium-like platinum-group elements, IPGE) behave as compatible elements during partial melting and fractional crystallization, whereas Rh, Pt, and Pd (the palladium- like platinum-group elements, PPGE) behave as incompatible elements [55–57]. Thus, the identified grains have been subdivided into two groups of PGMs according to their composition: the PPGE PGMs and the IPGE PGMs (Figure4a). The PPGE PGMs are considerably more abundant (93.66%, n = 2495) than the IPGE PGMs which account for 6.34% (n = 169) of the total PGMs identified. The PGMs in which Pt is the dominant metal represent 76.20% (n = 2030) of the total population, whilst Pd-rich minerals correspond to 16.74% (n = 446) of the PGMs. As a result, Pt- and Pd-rich minerals clearly dominate the PGM population (representing nearly 93%). In contrast, Rh-rich PGMs are rarely abundant constituting 0.71% (n = 19) of the identified grains (Figure4b). Within the IPGE PGMs population, IPGE alloys are identified (Figure3) and include Os-dominated, Ir- dominated and Ru-dominated alloys as well as rutheniridosmine. As shown in Figure4b, Os is the dominant metal in 2.40% (n = 64) of the PGMs closely followed by Ru (2.06%, n = 55) and Ir (1.88%, n = 50). Anduoite (Figure5d), irarsite (Figure5e), laurite (Figure5f,g), erlichmanite (Figure5h), and Ir-Ru-rich osmium (Figure5i) are IPGE PGM species variably abundant in the studied till samples (Figure3; Table1). MineralsMinerals 20212021, 11, 11, ,x 264 FOR PEER REVIEW 9 of 28 9 of 28

FigureFigure 3. Major3. Major platinum-group platinum-group mineral mineral (PGM) (PGM) species recoveredspecies recovered from till in from this study till in and this their study and their relativerelative abundance. abundance. A total A total of 58 differentof 58 different PGM species PGM have species been have identiﬁed. been identified.

TableTable 1. List1. List of rare of rare PGMs PGMs identiﬁed identified by ARTGold by ARTGoldTM in theTM studied in the till studied samples. till These samples. PGM species These PGM spe- arecies grouped are grouped as “Others” as “ inOthers” Figure3 .in Figure 3.

PGMPGM FormulaFormula n. * n. * PGMPGM Formula n.Formula n. VysotskiteVysotskite (Pd,Ni)S(Pd,Ni)S 11 Zvyagintsevite11 Zvyagintsevite Pd3Pb 3 Pd3Pb 3 AtheneiteAtheneite (Pd,Hg)3As(Pd,Hg)3As 11 Arsenopalladinite11 Arsenopalladinite (Pd8(As,Sb)3 (Pd8(As,Sb)3 2 2 Hollingworthite (Rh,Pt,Pd)AsS 10 Chrisstanleyite Ag2Pd3Se4 2 Hollingworthite (Rh,Pt,Pd)AsS 10 Chrisstanleyite Ag2Pd3Se4 2 Tatyanaite (Pt,Pd,Cu)9Cu3Sn4 10 Iridarsenite (Ir,Ru)As2 2 VincentiteTatyanaite Pd3As(Pt,Pd,Cu)9Cu3Sn4 10 Palarstanide10 Iridarsenite Pd5(Sn,As)2 2(Ir,Ru)As2 2 AnduoiteVincentite (Ru,Os)As2Pd3As 7 10 Palladseite Palarstanide Pd17Se15 Pd5(Sn,As)2 2 2 MitrofanoviteAnduoite Pt3Te4(Ru,Os)As2 7 Rustenburgite7 Palladseite (Pt,Pd)3Sn 2Pd17Se15 2 Pd-dominated alloys Pd 6 Atokite [Pd,Pt)3Sn] 1 WeishaniteMitrofanovite Pd(±Pt)AuHgPt3Te4 6 Kingstonite7 Rustenburgite [Rh,Ir,Pt)3S4] 1(Pt,Pd)3Sn 2 Pd-dominatedOsarsite alloys (Os,Ru)AsS Pd 66 Kotulskite Atokite (Pd(Te,Bi) 1[Pd,Pt)3Sn] 1 HongshiiteWeishanite PtCuPd(±Pt)AuHg 6 Merenskyite6 Kingstonite(Pd,Pt)(Te,Bi)2 [Rh,Ir,Pt)3S4]1 1 PaoloviteOsarsite Pd2Sn(Os,Ru)AsS 5 Minakawaite6 Kotulskite RhSb 1(Pd(Te,Bi) 1 Cabriite Pd2CuSn 5 Padmaite PdBiSe 1 StannopalladiniteHongshiite (Pd,Cu)3Sn2PtCu 4 Palladobismutharsenide6 MerenskyitePd2(As,Bi) (Pd,Pt)(Te,Bi)2 1 1 TaimyritePaolovite (Pd,Cu,Pt)3SnPd2Sn 4 Plumbopalladinite5 Minakawaite Pd3Pb2 1 RhSb 1 NaldretteiteCabriite Pd2SbPd2CuSn 4 Sudovikovite5 Padmaite PtSe2 1 PdBiSe 1 Niggliite PtSn 3 Telargpalite (Pd,Ag)3Te 1 Stannopalladinite (Pd,Cu)3Sn2 4 Palladobismutharsenide Pd2(As,Bi) 1 Omeiite (Os,Ru)As2 3 Ungavaite Pd4Sb3 1 RuarsiteTaimyrite (Ru,Os)AsS(Pd,Cu,Pt)3Sn 3 4 Plumbopalladinite Pd3Pb2 1 * NumberNaldretteite of grains. Pd2Sb 4 Sudovikovite PtSe2 1 Niggliite PtSn 3 Telargpalite (Pd,Ag)3Te 1 Omeiite (Os,Ru)As2 3 Ungavaite Pd4Sb3 1 Ruarsite (Ru,Os)AsS 3 * Number of grains.

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FigureFigure 4.4. AbundanceAbundance ofof platinum-groupplatinum-group mineralsminerals (PGMs)(PGMs) recoveredrecovered inin heavyheavy mineralmineral super-concentratessuper-concentrates obtainedobtained afterafter processingFigureprocessing 4. Abundance tilltill samples.samples. of platinum-group ((aa)) PGMsPGMs areare divideddivided minerals intointo (PGMs) twotwo majormajor recovere groups:groups:d in palladium-like palladium-likeheavy mineral platinum-groupsuper-coplatinum-groupncentrates elementselements obtained (PPGE)(PPGE) after PGMs, whose compositions are dominated by Pd, Pt, and/or Rh, and the iridium-like platinum-group elements (IPGE) processingPGMs, whose till compositionssamples. (a) PGMs are dominated are divided by into Pd, Pt,two and/or major Rh,groups: and thepalladium-like iridium-like platinum-group platinum-group elements elements (PPGE) (IPGE) PGMs, in which Ir, Os and/or Ru are major components. (b) PGMs in which Pt is the dominant metal represent 76.20% (n PGMs, whose in which compositions Ir, Os and/or are Ru dominated are major by components. Pd, Pt, and/or (b) PGMsRh, and in the which iridium-like Pt is the dominantplatinum-group metal representelements 76.20%(IPGE) = 2030), whilst Pd is the dominant metal for 16.74% (n = 446) of the PGM population recovered from till in this study (n = (PGMs,n = 2030), in which whilst Ir, Pd Os is and/or the dominant Ru are major metal components. for 16.74% (n (b=) 446)PGMs of in the which PGM Pt population is the dominant recovered metal from represent till in this76.20% study (n =2664). 2030), whilst Pd is the dominant metal for 16.74% (n = 446) of the PGM population recovered from till in this study (n = (n = 2664). 2664).

Figure 5. A selection of backscatter electron (BSE) images of precious metal minerals (PMM) including Ag-bearing phases Figure(a–c) and 5. A minerals selection of of the backscatter iridium-like electron platin (BSE)um-group images elements of precious (IPGE-PGM) metal minerals identifi (PMM)ed in the including till samples Ag-bearing processed phases and Figure 5. A selection of backscatter electron (BSE) images of precious metal minerals (PMM) including Ag-bearing phases (analyzeda–c) and mineralsby IOS-SG. of (thea) Native iridium-like silver (Ag);platin (um-groupb) Acanthite elements (Ag2S); (IPGE-PGM)(c) Hessite (Ag identifi2Te); (ded) Anduoitein the till ((Ru,Os)Assamples processed2); (e) Irarsite and (((Ir,Ru,Rh,Pt)AsS);a–c) and minerals of(f, theg) Laurite iridium-like (RuS2); platinum-group (h) Erlichmanite elements (OsS2); ( (IPGE-PGM)i) An Ir-Ru-rich identiﬁed osmium in grain. the till samples processed and analyzed by IOS-SG. (a) Native silver (Ag); (b) Acanthite (Ag2S); (c) Hessite (Ag2Te); (d) Anduoite ((Ru,Os)As2); (e) Irarsite analyzed by IOS-SG. (a) Native silver (Ag); (b) Acanthite (Ag S); (c) Hessite (Ag Te); (d) Anduoite ((Ru,Os)As ); (e) Irarsite ((Ir,Ru,Rh,Pt)AsS); (f,g) Laurite (RuS2); (h) Erlichmanite (OsS22); (i) An Ir-Ru-rich2 osmium grain. 2 ((Ir,Ru,Rh,Pt)AsS); (f,g) Laurite (RuS2); (h) Erlichmanite (OsS2); (i) An Ir-Ru-rich osmium grain.

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Figure 6. A selection of backscatter electron (BSE) images of platinum-group minerals identified in the till samples. (a) Figure 6. A selection of backscatter electron (BSE) images of platinum-group minerals identiﬁed in the till samples. Braggite ((Pt,Pd,Ni)S); (b) Cooperite (PtS); (c) Moncheite ((Pt,Pd)(Te,Bi)2); (d) Stillwaterite (Pd8As3); (e) Mertieite-II (a) Braggite ((Pt,Pd,Ni)S); (b) Cooperite (PtS); (c) Moncheite ((Pt,Pd)(Te,Bi) ); (d) Stillwaterite (Pd As ); (e) Mertieite-II (Pd8(Sb,As)3); (f) Taimyrite ((Pd,Cu,Pt)3Sn); (g) Cabriite (Pd2SnCu); (h) Isomertieite2 (Pd11Sb2As2); (i)8 Paolovite3 (Pd2Sn); (j) f g h i (PdPotarite8(Sb,As) (PdHg);3); ( ) ( Taimyritek) Stibiopalladinite ((Pd,Cu,Pt) (Pd3Sn);5Sb2 (); )(l Cabriite) Platinum (Pd (Pt).2SnCu); ( ) Isomertieite (Pd11Sb2As2); ( ) Paolovite (Pd2Sn); (j) Potarite (PdHg); (k) Stibiopalladinite (Pd5Sb2); (l) Platinum (Pt). 3.2. Chemical Signature of Abundant Platinum-Group Minerals 3.2. Chemical Signature of Abundant Platinum-Group Minerals 3.2.1. Sperrylite 3.2.1. Sperrylite Sperrylite corresponds to more than half of the PGM population recovered from the Sperrylite corresponds to more than half of the PGM population recovered from the studied till samples. The box and whisker plots in Figure 7 show the range of variation in studied till samples. The box and whisker plots in Figure7 show the range of variation in the composition of Pt and As and their substituting elements in 1488 analyzed sperrylite the composition of Pt and As and their substituting elements in 1488 analyzed sperrylite grains, including some As-poor and Pt-rich grains probably representing altered grains. grains, including some As-poor and Pt-rich grains probably representing altered grains. TheseThese analyses are semi-quantit semi-quantitativeative (standardless), (standardless), thus, thus, they they have have been been normalized normalized to to100% 100% and and with with a detection a detection limit limit in the in theorder order of 1% of 1%for most for most metals. metals. As a Asresult, a result, the indi- the individualvidual concentration concentration of the of substituting the substituting elements elements does doesnot exceed not exceed 10 wt% 10 (Figure wt% (Figure 7). How-7 ). However,ever, some some of these of theseelements elements are only are measured only measured in a few in grains a few grains(1 ≥ Os, (1 Hg,≥ Os,Pb, Hg,Ag, Au, Pb, Ag,Ru, Au,Te, Ir, Ru, Sb, Te, Cu, Ir, and Sb, Pd Cu, ≤ and25), whereas Pd ≤ 25), sperrylite whereas contains sperrylite Rh contains (81 grains), Rh Ni (81 (105 grains), grains), Ni and S (387 grains) more often. The proportions of As and Pt are not necessarily

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(105 grains), and S (387 grains) more often. The proportions of As and Pt are not necessarily stoichiometricstoichiometric and and may may vary vary over over a wide a wide range, range, though though 50% 50% of the of grains the grains contain contain≤ 40 wt% ≤ 40 As,wt% while As, while the median the median concentration concentration for Pt for in sperrylitePt in sperrylite is 59 wt%. is 59 Anwt%. average An average of 4.9 of wt% 4.9 Auwt% is alsoAu is measured also measured in 3 sperrylite in 3 sperrylite grains grains (Figure (Figure7). 7).

Figure 7. Multi-element box and whisker plot showing elemental variation (in wt%) in the compo- Figure 7. Multi-element box and whisker plot showing elemental variation (in wt%) in the composi- sition of 1488 sperrylite grains. Whiskers represent the range of minimum and maximum values. tion of 1488 sperrylite grains. Whiskers represent the range of minimum and maximum values. The The short line within each box represents the median value, which separates the box into two shortparts line (i.e., within lower each25th–50th box represents percentile the group median and value,upper which50th–75th separates percentile). the box The into n below two parts the (i.e.,axis: lowernumber 25th–50th of grains percentile containing group the andelement upper of 50th–75thinterest. percentile). The n below the axis: number of grains containing the element of interest. 3.2.2. Braggite-Cooperite-Vysotskite Series 3.2.2. Braggite-Cooperite-Vysotskite Series Following sperrylite, cooperite and braggite (Figure 6a,b) are ranked the most abun- Following sperrylite, cooperite and braggite (Figure6a,b) are ranked the most abundant PGMs in the till samples. These minerals along with vysotskite form an isomorphous dant PGMs in the till samples. These minerals along with vysotskite form an isomor- solid-solution series ((Pt,Pd,Ni)S), though cooperite is ideally PtS, braggite (Pd,Pt)S, and phous solid-solution series ((Pt,Pd,Ni)S), though cooperite is ideally PtS, braggite (Pd,Pt)S, vysotskite PdS [8,58]. Some studies have already established chemical classification and vysotskite PdS [8,58]. Some studies have already established chemical classification schemes for differentiating among the PGE-bearing nickeloan sulfides [25,58]. However, schemes for differentiating among the PGE-bearing nickeloan sulfides [25,58]. However, the suggested classifications mostly consider differences in the content of major constitu- the suggested classifications mostly consider differences in the content of major constituents (Pd,ents Pt, (Pd, S, ±Pt,Ni), S, and±Ni), therefore, and therefore, they do they not do reflect not thereflect variable the variable composition composition of any substi-of any tutingsubstituting elements elements (e.g., Cu, (e.g., Ru, Cu Rh,, Ru, Au, Rh, Ag) Au, in Ag) the in given the given minerals. minerals. These These classifications classifications do notdo not also also define define the the range range of elementalof elemental compositions compositions that that can can be be used used for for differentiating differentiating amongamong a a Pd-rich Pd-rich cooperite cooperite and and a a Pt-rich Pt-rich braggite, braggite, as as well well as as braggite braggite and and Pt-rich Pt-rich vysotskite vysotskite (Figure(Figure8 8).). Hence, Hence, Figure Figure8 8yields yields a a classification classification diagram diagram for for PPGE PPGE PGMs PGMs satisfying satisfying the the followingfollowing conditioncondition ((Pt((Pt at.at. + + Pd Pd at. at. + +Ni Ni at. at. + others + others at.)/S at.)/S at. ~1) at. to ~1) discriminate to discriminate between betweencooperite, cooperite, braggite, braggite, and vysotskite and vysotskite based based on their on their compositional compositional variation. variation. Analyses Analyses by byCabri Cabri [25] [25 are] are included included to help to help in defining in defining the thefields. fields. Given Given that the that limit the limitbetween between coop- cooperiteerite and andbraggite, braggite, and andbetween between braggite braggite and vy andsotskite vysotskite are not are explicitly not explicitly specified specified in the inliterature, the literature, they were they set were based set on based how on they how cluster. they cluster.As a result, As athe result, limit thebetween limit betweencooperite cooperiteand braggite and braggiteis fixed at is fixed75 wt% at 75Pt, wt% which Pt, means which means240 analyses 240 analyses having having Pt > 75 Pt wt% > 75 corre- wt% correspondspond to cooperite to cooperite (Figure (Figure 8). Similarly,8). Similarly, the limit the limit between between vysotskite vysotskite and andbraggite braggite is fixed is fixedat 12.5 at 12.5wt% wt%Pt suchPt such that that227 227analyses analyses satisfying satisfying the theaforementioned aforementioned condition condition and andalso alsohaving having 12.512.5 wt% wt% > Pt > < Pt 75 < wt% 75 wt% correspondcorrespond to braggite, to braggite, whereas whereas 11 analyses 11 analyses having having Pt < Pt12.5 < 12.5wt% wt% are clustered are clustered as vysotskite as vysotskite (Figure (Figure 8). 8). ElementalElemental variationvariation in the composition composition of of braggite, braggite, cooperite cooperite and and vysotskite vysotskite is isillus- il- lustratedtrated by bya box a box and and whisker whisker plots plots in Figure in Figure 9a–c.9a–c. Although Although forming forming a solid asolid solution solution series, series,and within and within a fixed a range fixed rangeof Pt composition, of Pt composition, these PGMs these PGMsshow a show variety a variety range rangeof values of valuesfor Pd, for Ni Pd, and Ni substituting and substituting elements. elements. For instance, For instance, whereas whereas braggite braggite is composed is composed of 10 < ofPd10 < <64 Pd wt% < 64 (Figure wt% (Figure 9a), and9a), cooperite and cooperite consists consists of 1.4 of ≤ 1.4Pd ≤≤ 12Pd wt%≤ 12 (Figure wt% (Figure 9b), the9b), Pd contents in vysotskite vary from > 55 to < 78 wt%. In addition, the range of Ni values,

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the Pd contents in vysotskite vary from > 55 to < 78 wt%. In addition, the range of Ni values, which was measured in ~31.0% of braggite grains and ~8.0% of cooperite grains iswhichwhich as follows: was was measured measured Ni ≥ in0.5 ~ in 31.0 ~ to 31.0 %≤ of %16.0 braggite of braggite wt% grains for grains braggite and ~and 8.0 %~ and 8.0 of cooperite% > of 0.7 cooperite to grains < 3.5 grains is wt% as follows: is for as follows: cooperite (NiFigureNi ≥ 0.5≥ 0.5 to9 a,b).to≤ 16.0 ≤ 16.0 Vysotskitewt% wt% for forbraggite braggite commonly and and > 0.7 > (82.0%)to 0.7 < 3.5to < wt% 3.5 contains wt%for cooperite for of cooperite 0.9 (Figure < Ni (Figure <9a,b). 12.5 Vysotskite9a,b). wt% Vysotskite (Figure 9c). Variablecommonlycommonly contents (82.0%) (82.0%) contains of contains Sb, Ru, of 0.9 of As <0.9 Ni Se, < < Ni Cu,12.5 < 12.5 andwt% wt% Rh(Figure are(Figure 9c). measured Variable 9c). Variable in contents braggite contents of Sb, and Ru,of cooperiteSb, Ru, (FigureAsAs Se, Se, Cu,9 Cu,a,b), and and whilstRh Rh are are measured 19 measured braggite in braggite grains in braggite also and consistandcooperite cooperite of (Figure 1.0 <(Figure Ag 9a,b), < 9a,b), 3.3whilst wt%, whilst 19 braggite and 19 5 braggite cooperite grains also consist of 1.0 < Ag < 3.3 wt%, and 5 cooperite grains contain in average ~ 4.0 grainsgrains contain also consist in average of 1.0 < ~4.0Ag < wt% 3.3 wt%, Au. and Vysotskite 5 cooperite results grains also contain include in oneaverage analysis ~ 4.0 at wt% Au. Vysotskite results also include one analysis at 6.7 wt% Au and two analyses av- 6.7wt% wt% Au. Au Vysotskite and two analysesresults also averaging include one ~2 wt%analysis Ag (Figureat 6.7 wt%9c). Au and two analyses av- eragingeraging ~ 2~ wt% 2 wt% Ag Ag (Figure (Figure 9c). 9c).

Figure 8. This graph classifies the Pt- and Pd-bearing sulfides whose compositions satisfy the fol- Figure 8. This graph classifies the Pt- and Pd-bearing sulfides whose compositions satisfy the lowing condition ((Pt at. + Pd at. + Ni at. + others at.)/ S at. ~1). The elemental values are presented Figure 8. This graph classifies the Pt- and Pd-bearing sulfides whose compositions satisfy the fol- followingin atomic proportions condition (at. ((Pt). The at. analyses + Pd at. from + Ni Cabri at. [25] + others are included. at.)/ S This at. ~1).classification The elemental sets a values are lowing condition ((Pt at. + Pd at. + Ni at. + others at.)/ S at. ~1). The elemental values are presented presentedlimit for vysotskite in atomic at Pt proportions < 12.5 wt% and (at.). a Thelimit analysesfor the cooperite from Cabri at Pt > [25 75.0] are wt%. included. Braggite Thisforms classification a in atomic proportions (at.). The analyses from Cabri [25] are included. This classification sets a setsfield abetween limit for the vysotskite given limits at (12.5 Pt < wt% 12.5 > wt%Pt < 75.0 and wt%). a limit for the cooperite at Pt > 75.0 wt%. Braggite limit for vysotskite at Pt < 12.5 wt% and a limit for the cooperite at Pt > 75.0 wt%. Braggite forms a formsfield abetween field between the given the limits given (12.5 limits wt% (12.5 > Pt wt% < 75.0 > Ptwt%). < 75.0 wt%).

Figure 9. Multi-element box and whisker plots of elemental variation (in wt%) in (a) braggite, (b) cooperite, and (c) vysotskite grains. Whiskers represent the range of minimum and maximum values. The short line within each box represents the median value, which separates the box into two parts (i.e., lower 25th–50th percentile group and upper 50th– Figure75thFigure 9.percentile).Multi-element 9. Multi-element The n below box box and the and whisker axis: whisker number plots plotsof of grains elemental of elementalcontaining variation variationthe element (in wt%) (in of inwt%) interest. (a) braggite,in (a) braggite, (b) cooperite, (b) cooperite, and (c) vysotskiteand (c) grains.vysotskite Whiskers grains. represent Whiskers the represent range of the minimum range of minimu and maximumm and maximum values. The values. short The line short within line within each box each represents box repre- the sents the median value, which separates the box into two parts (i.e., lower 25th–50th percentile group and upper 50th– median value, which separates3.2.3. the Isomertieite-Mertieite box into two parts (i.e., Series lower 25th–50th percentile group and upper 50th–75th percentile). 75th percentile). The n below the axis: number of grains containing the element of interest. The n below the axis: number ofMertieite-II grains containing (Pd8(Sb,As) the element3; Figure of interest. 6e), mertieite-I (Pd11(Sb,As)4), and isomertieite (Pd11Sb2As2; Figure 6h) are respectively the fifth, eighth, and ninth most abundant PGMs 3.2.3. Isomertieite-Mertieite Series 3.2.3.identified Isomertieite-Mertieite in the studied till samples Series (Figure 3). They are members of palladium arseno- antimonidesMertieite-II and crystallize (Pd8(Sb,As) in different3; Figure crysta 6e),llographic mertieite-I systems. (Pd11 As(Sb,As) their4 ),stoichiometric and isomertieite Mertieite-II (Pd (Sb,As) ; Figure6e), mertieite-I (Pd (Sb,As) ), and isomertieite (Pd11Sb2As2; Figure 6h)8 are respectively3 the fifth, eighth, and 11ninth most4 abundant PGMs (Pdidentified11Sb2As 2in; Figurethe studied6h) are till respectively samples (Figur thee ﬁfth, 3). They eighth, are andmembers ninth of most palladium abundant arseno- PGMs identiﬁedantimonides in the and studied crystallize till samplesin different (Figure crysta3llographic). They are systems. members As oftheir palladium stoichiometric arseno-

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Minerals 2021, 11, x FOR PEER REVIEWratios are too close to be efficiently discriminated from the semi-quantitative14 of 28 analysis pro- antimonides and crystallize in different crystallographic systems. As their stoichiometric duced by the routine, it is difficult to discriminate between these mineral species. Despite ratios are too close to be efficiently discriminated from the semi-quantitative analysis pro- that mertieite-I and isomertieite are presented as dimorphous species, it appears from ducedratiosavailable are by too thedata close routine, that to be mertieite-I effi itciently is difficult discriminated contains to discriminate slightly from the less semi-quantitative between As (6.6 wt% these in analysis mineral average) pro- species. and slightly Despite duced by the routine, it is difficult to discriminate between these mineral species. Despite thatmore mertieite-I Sb (13.7 wt% and isomertieitein average) compared are presented to isomertieite as dimorphous (average species, As = it 8.6 appears wt% and from Sb avail- = that mertieite-I and isomertieite are presented as dimorphous species, it appears from able12.7 data wt%; that Figures mertieite-I 10 and contains 11). On slightlythe other less hand, As (6.6the composition wt% in average) of mertieite-II and slightly grains more is Sb (13.7available wt% data in average)that mertieite-I compared contains to slightly isomertieite less As (6.6 (average wt% in As average) = 8.6 and wt% slightly and Sb = 12.7 wt%; moredistinctly Sb (13.7 enriched wt% in average) in Sb and compared impoverished to isomertieite in As, (average placing As these= 8.6 wt% in a and distinctive Sb = field in Figures12.7Figure wt%; 1010. Figures andThis 11 10discriminant). and On 11). the On other the diagram other hand, hand, illustrates the the composition composition how the of composition mertieite-II mertieite-II grains of grains the is major is distinctly ele- enricheddistinctlyments (Pd, enriched in Sb, Sb andAs) in andSb impoverished and substituting impoverished in constitu As, in As, placingents placing forms these these a inloosein a distinctive distinctive continuum field field among in in Figurethe pal- 10. ThisFigureladium discriminant 10. arseno-antimonides This discriminant diagram diagram illustrates species. illustrates To how establish how the compositionthe the composition limits between of of the the major majorthe fields elementsele- of isomer- (Pd, Sb, As)mentstieite, and (Pd, mertieite-I, substituting Sb, As) and and substituting constituents mertieite-II, constitu formsthe entsdata a forms loosefrom a Cabri continuumloose continuum [25] have among amongbeen the incorporated the palladium pal- in arseno- the ladium arseno-antimonides species. To establish the limits between the fields of isomer- antimonidesgraph as reference. species. To establish the limits between the fields of isomertieite, mertieite-I, andtieite, mertieite-II, mertieite-I, and the mertieite-II, data from the Cabri data [from25] have Cabri been[25] have incorporated been incorporated in the in graph the as reference. graph as reference.

Figure 10. The graph depicts the classification of palladium arseno-antimonides species of the Figure 10. The graph depicts the classiﬁcation of palladium arseno-antimonides species of the platinum-groupFigure 10. The minerals graph depictsbelonging the to classificationthe isomertieite-mertieite of palladium series. arseno-antimonides The elemental values species are of the platinum-grouppresented in atomic minerals proportions belonging (at.). Data from to the Ca isomertieite-mertieitebri [25] have been included series. to help Theestablishing elemental values are platinum-group minerals belonging to the isomertieite-mertieite series. The elemental values are presentedthe limits between in atomic the isomertieite, proportions me (at.).rtieite-I, Data andfrom mertieite-II Cabri fields. [25] have been included to help establishing presented in atomic proportions (at.). Data from Cabri [25] have been included to help establishing thethe limits limits between between thethe isomertieite,isomertieite, me mertieite-I,rtieite-I, and and mertieite-II mertieite-II fields. ﬁelds.

Figure 11. Multi-element box and whisker plots showing elemental variation (in wt%) in the composition of (a) isomertieite, (b) mertieite-I, and (c) mertieite-II grains recovered from tills. Whiskers represent the range of minimum and maximum values. The short line within each box represents the median value, which separates the box into two parts (i.e., lower

FigureFigure 11. 11.Multi-element Multi-element box box and and whisker whisker plots plots showing showing elemental elemental variation variation (in(in wt%)wt%) inin thethe compositioncomposition of ( a) isomertie- isomertieite, (b)ite, mertieite-I, (b) mertieite-I, and (andc) mertieite-II (c) mertieite-II grains grains recovered recovered from from tills. tills. Whiskers Whiskers represent represent thethe range of of minimum minimum and and maximum maximum values. The short line within each box represents the median value, which separates the box into two parts (i.e., lower values. The short line within each box represents the median value, which separates the box into two parts (i.e., lower 25th–50th percentile group and upper 50th–75th percentile). The n below the axis: number of grains containing the element of interest. Minerals 2021, 11, x FOR PEER REVIEW 15 of 28

Minerals25th–50th2021, 11, 264 percentile group and upper 50th–75th percentile). The n below the axis: number of grains containing the element15 of 28 of interest.

Mertieite-II analyses are characterized by (Sb + Te + Ag) > 20 wt% and (As at. + S at.) Mertieite-II analyses are characterized by (Sb + Te + Ag) > 20 wt% and (As at. + S / (Sb at. + Te at. + Ag at.) < 0.5 (Figure 10). In contrast, mertieite-I and isomertieite analyses at.) / (Sb at. + Te at. + Ag at.) < 0.5 (Figure 10). In contrast, mertieite-I and isomertieite are characterized by (Sb + Te + Ag) < 20 wt%. The difference between these two species analyses are characterized by (Sb + Te + Ag) < 20 wt%. The difference between these two lies essentially on the As / Sb ratio where mertieite-I are (As at. + S at.) / (Sb at. + Te at. + species lies essentially on the As / Sb ratio where mertieite-I are (As at. + S at.) / (Sb at.Ag + at.) Te < at. 0.8 + while Ag at.) isomertieite < 0.8 while are isomertieite (As at. + S at.) are / (As(Sb at. at. + + Te S at. at.) + /Ag (Sb at.) at. > 0.8 + Te (Figure at. + Ag10). at.)Box > and 0.8 whisker (Figure 10 plots). Box of isomertieite, and whisker mertieit plots ofe-I isomertieite, and mertieite-II, mertieite-I show andhow mertieite-II, these series showcontain how variable these series amounts contain of Ag variable and Te amounts that most of Ag likely and substitute Te that most for likelySb and substitute variable foramounts Sb and of variable Ni, Pt, Cu, amounts Au, and of Ni,Hg that Pt, Cu, seems Au, to and substitute Hg that for seems Pd (Figure to substitute 11a-c). forWithin Pd (Figurethe aforementioned 11a–c). Within As the+ Sb aforementioned limits of these species, As + Sb Pd limits contents of these varies species, from ≥ Pd65.8 contents to < 85.0 varieswt% in from isomertieite≥ 65.8 to(Figure < 85.0 11a), wt% ≥ in66.2 isomertieite to < 83.2 wt% (Figure in mertieite-I 11a), ≥ 66.2(Figure to <11b) 83.2 and wt% > 59.0 in mertieite-Ito < 78.0 wt% (Figure in mertieite-II 11b) and > (Figure 59.0 to 11c). < 78.0 Among wt% in the mertieite-II 161 palladium (Figure arseno-antimonides 11c). Among the 161species palladium analyzed, arseno-antimonides two mertieite-I grains species also analyzed, contain two 5.0 and mertieite-I 7.7 wt% grains Au and also three contain isomer- 5.0 andtieite 7.7 reported wt% Au 4.5 and < Au three < 12.5 isomertieite wt%. reported 4.5 < Au < 12.5 wt%.

3.2.4.3.2.4. Laurite-ErlichmaniteLaurite-Erlichmanite SeriesSeries

TheThe IPGEIPGE PGMPGM populationpopulation mostlymostly consistsconsists ofof Ru–OsRu–Os disulﬁdesdisulfides of of the the laurite laurite (RuS (RuS22;; FigureFigure5 f,g)5f,g) and and erlichmanite erlichmanite (OsS (OsS22;; FigureFigure5 h)5h) series, series, which which are are ranked ranked as as the the sixth sixth most most abundantabundant PGMPGM groupgroup inin the the database database (72 (72 grains: grains: Figure Figure3 ).3). The The Os-Ru-(Rh Os-Ru-(Rh + + Ir) Ir) ternary ternary plotplot inin FigureFigure 12 12 shows shows that that the the composition composition ofof these these 72 72 sulﬁde sulfide grains grains varies varies from from pure pure erlichmaniteerlichmanite throughthrough Ru-richRu-rich erlichmanite,erlichmanite, Os-bearingOs-bearing lauritelaurite toto laurite.laurite. However,However, pure pure lauritelaurite has has not not been been found. found. AA relativelyrelatively Ir-richIr-rich erlichmaniteerlichmanite waswas alsoalso detecteddetected (Figure(Figure 1212).). BoxBox andand whiskerwhisker plotsplots indicate indicate that that the the Os Os content content varies varies from from≥ ≥11.5 11.5 to to≤ ≤ 44.244.2 wt%wt%in in lauritelaurite (Figure(Figure 13 13a),a),whilst whilstthe the range range of of Os Os for for erlichmanite erlichmanite is is from from≥ ≥35.2 35.2to to< < 78.078.0wt% wt% (Figure(Figure 13 13b).b). Both minerals minerals may may contain contain < <10 10 wt wt%% Ni Ni and/or and/or As. As.Erlichmanite Erlichmanite is also is char- also characterizedacterized by 4.9 by ≤ 4.9 Ir <≤ 39.0Ir < wt%, 39.0 6.1 wt%, ≤ Ru 6.1 ≤ ≤21.4Ru wt%≤ 21.4 and wt%16.3 ≤ and S ≤ 16.329.0 ≤wt%,S ≤ whereas29.0 wt%, the whereasranges for the laurite ranges are for as laurite follows: are 3.1 as follows:≤ Ir < 20.5 3.1 wt%,≤ Ir 18.5 < 20.5 ≤ Ru wt%, ≤ 42.8 18.5 wt%≤ Ru and≤ 42.825.6 wt%≤ S ≤ and38.0 25.6 wt%≤ (FigureS ≤ 38.0 13a,b). wt% (Figure 13a,b).

FigureFigure 12. 12.The The Os-Ru-(RhOs-Ru-(Rh ++ Ir)Ir) diagramdiagram illustratesillustrates thethe compositionalcompositionaldistribution distributionof of 72 72 PGM PGM grains grains belongingbelonging to to the the laurite-erlichmanite laurite-erlichmanite series. series. The The elemental elemental values values are ar presentede presented inatomic in atomic proportions (at.). Data from Cabri [25] have been included to help establish the limits between laurite and erlichmanite ﬁelds.

Minerals 2021, 11, x FOR PEER REVIEW 16 of 28

Minerals 2021, 11, 264 16 of 28 proportions (at.). Data from Cabri [25] have been included to help establish the limits between laurite and erlichmanite fields.

FigureFigure 13. 13.Multi-element Multi-element boxbox andand whiskerwhisker plotsplots showing elemental variation (in wt%) in in the the com- com- positionposition ofof ((aa)) lauritelaurite andand ((bb)) erlichmaniteerlichmanite grains. Whiskers Whiskers re representpresent the the range range of of minimum minimum and and maximum values. The short line within each box represents the median value, which separates the maximum values. The short line within each box represents the median value, which separates the box into two parts (i.e., lower 25th–50th percentile group and upper 50th–75th percentile). The n box into two parts (i.e., lower 25th–50th percentile group and upper 50th–75th percentile). The n below the axis: number of grains containing the element of interest. below the axis: number of grains containing the element of interest. 3.2.5. Potarite 3.2.5. Potarite Potarite is a relatively abundant palladium amalgam (2.14% of the population, n = Potarite is a relatively abundant palladium amalgam (2.14% of the population, n = 57) 57) recovered from till. Although it is ideally composed of Hg and Pd, various metals (e.g., recovered from till. Although it is ideally composed of Hg and Pd, various metals (e.g., Bi, Bi, Sb, Se, Cu, Ag, and Au; Figure 14a) can substitute for either of the major components. Sb, Se, Cu, Ag, and Au; Figure 14a) can substitute for either of the major components. As theAs boxthe andbox whiskerand whisker plots plots show, show, Hg content Hg content varies overvaries a wideover rangea wide (≥ range18.4 to (≥≤18.471.2 to wt% ≤71.2), thoughwt%), though 50% of 50% the grainsof the containgrains contain≥ 64.8 ≥ wt% 64.8 Hg wt% (Figure Hg (Figure 14a). A14a). large A variationlarge variation is also is observedalso observed for Pd for such Pd such that that the the minimum minimu andm and maximum maximum values values measured measured are are 21.6 21.6 and and 78.778.7 wt%wt%,, respectively,respectively, whilst this variation variation fo forr 50% 50% of of the the grains grains varies varies from from 30.8 30.8 to 34.1 to 34.1wt% wt% Pd (FigurePd (Figure 14a). 14 Sucha). Such kind kind of variation of variation is typical is typical of alloys of alloys that thatare not are ruled not ruled by stoi- by stoichiometricchiometric ratios. ratios. The The Cu Cu content content can can reach reach up up to to17.4 17.4 wt%. wt%. Moreover, Moreover, Au Au abundances abundances in inAu-bearing Au-bearing potarite potarite range range between between ≥ 3.8≥ 3.8 to ≤ to 46.6≤ 46.6 wt%, wt%, as measured as measured in 14 in grains 14 grains such such that thatin Hg-(Pd in Hg-(Pd + Cu)-Au + Cu)-Au ternary ternary plot, plot, a afield ﬁeld formed formed by by Au-rich Au-rich potarite isis distinguishabledistinguishable (Figure(Figure 14 14b).b). The The most most Au-rich Au-rich potarite potarite (46.65 (46.65 wt% wt% Au, Au, 21.6 21.6 wt% wt% Pd andPd and 31.7 31.7 wt% wt% Hg) hasHg) beenhas been recovered recovered from from till samples till samples collected collect fromed from Nunavik Nunavik area area (Northern (Northern Quebec; Quebec; Canada; Can- Figureada; Figure 15a) associated 15a) associated with abundantwith abundant amalgamated amalgamated gold grainsgold grains and cinnabar. and cinnabar.

3.3.3.3. GoldGold AssociationAssociation andand PolymineralicPolymineralic OccurrenceOccurrence of of Platinum-Group Platinum-Group Minerals Minerals Platinum-groupPlatinum-group minerals are are occasionally occasionally found found in in association association with with native native gold gold (Figure(Figure 15 15)) as as well well as as grains grains made made of of complex complex PGM PGM assemblages assemblages (Figure (Figure 16 16).). AsideAside ofof potarite,potarite, amongamong the other other 57 57 PGM PGM species species identified identified in inthe the till tillsamples, samples, seven seven species species have havebeen beenfound found associated, associated, or as intergrowths or as intergrowths with native with nativegold grains. goldgrains. The majority The majority of these ofbelong these to belong the isomertieite-mertieite to the isomertieite-mertieite series (Figure series 15b–d). (Figure 15Asb–d). a result, As ain result, three inobserva- three observations,tions, mertieite-I mertieite-I grains seem grains partly seem encaps partlyulated encapsulated by Au. Other by Au. Au Other-PGM Au-PGM associations associ- in- ationsclude includeisomertieite isomertieite and platinum and platinum inclusions inclusions in gold (Figure in gold 15d,h), (Figure fine-grained 15d,h), fine-grained vincentite vincentiteand atheneite and atheneiteattached to attached gold particles to gold (Figure particles 15e–g), (Figure and 15 e–g),a vysotskite and a vysotskite grain partly grain en- partlyclosedenclosed by gold (Figure by gold 15f). (Figure Figure 15f). 15i Figure shows 15 a ivery shows fine-grained a very fine-grained gold grain goldthat grown grain thaton the grown surface on theof a surfacesperrylite. of aFurthermore, sperrylite. Furthermore, in a limited number in a limited of particles, number polymineralic of particles, polymineralicaggregates of aggregatesPGMs are ofidentified PGMs are and identified characterized. and characterized. Two phases Twoaggregates phases aggregatesinclude: (1) include: (1) irarsite ((Ir,Ru,Rh,Pt)AsS) and sperrylite (Figure 16a); (2) Pt-rich irarsite and Ru-rich irarsite (Figure 16b); (3) rutheniridosmine (Ir,Os,Ru) with erlichmanite (Figure 16c);

Minerals 2021, 11, x FOR PEER REVIEW 17 of 28

Minerals 2021, 11, 264 17 of 28

Minerals 2021, 11, x FOR PEER REVIEWirarsite ((Ir,Ru,Rh,Pt)AsS) and sperrylite (Figure 16a); (2) Pt-rich irarsite and 17Ru-rich of 28 irarsite (Figure 16b); (3) rutheniridosmine (Ir,Os,Ru) with erlichmanite (Figure 16c); (4) ru- (4)theniridosmine rutheniridosmine (Ir,Os,Ru) (Ir,Os,Ru) with withplatarsite platarsite ((Pt,Rh,Ru)AsS) ((Pt,Rh,Ru)AsS) (Figure (Figure 16d); (5) 16 d);platarsite (5) platarsite and and sperrylite (Figure 16e), and (6) a three-phase PGM in which sperrylite is surrounding sperryliteirarsite ((Ir,Ru,Rh,Pt)AsS) (Figure 16e), and and (6) sperrylite a three-phase (Figure PGM 16a); (2)in whichPt-rich sperryliteirarsite and is Ru-rich surrounding irar- hol- hollingworthite ((Rh,Pt,Pd)AsS) and platarsite (Figure 16f). Sperrylite, comparing to the lingworthitesite (Figure 16b); ((Rh,Pt,Pd)AsS) (3) rutheniridosmine and platarsite (Ir,Os,Ru) (Fig withure 16f). erlichmanite Sperrylite, (Figure comparing 16c); (4) to ru-the other otherPGE-bearingtheniridosmine PGE-bearing minerals, (Ir,Os,Ru) minerals, appears with appears platarsite to be tomore ((Pt,Rh,Ru)AsS) be moreoften oftena component (Figure a component 16d); of polymineralic (5) of platarsite polymineralic and PGM ag- PGM aggregatesgregatessperrylite (Figure(Figure (Figure 16e),16a,e,f). 16a,e,f). and (6) a three-phase PGM in which sperrylite is surrounding hollingworthite ((Rh,Pt,Pd)AsS) and platarsite (Figure 16f). Sperrylite, comparing to the other PGE-bearing minerals, appears to be more often a component of polymineralic PGM aggregates (Figure 16a,e,f).

FigureFigure 14. 14.The The compositional compositional variation variation of of 57 57 potarite potarite grains grains is is illustrated illustrated in in ( a()a) box box and and whisker whisker plot, plot, and and ((bb)) anan AuAu at.–Hgat.–

at.–PdHg at.–Pd + Cu at. + Cu ternary at. ternary diagram. diagram. Whiskers Whiskers represent represent the the range range of of minimum minimum andand maximummaximum values. values. The The short short line line within within eachFigure box 14.represents The compositional the median variation value, ofwhich 57 potarite separates grains the is box illustrated into two in parts(a) box (i .e.,and lower whisker 25th–50th plot, and percentile (b) an Au groupat.– and each box represents the median value, which separates the box into two parts (i.e., lower 25th–50th percentile group and upperHg at.–Pd 50th–75th + Cu at. percentile). ternary diagram. The n Whiskersbelow the represent axis: number the range of gr ofains minimum containing and maximum the element values. of interest. The short The line coloredwithin poly- uppergoneach 50th–75th in bbox limits represents percentile). the field the for median TheAu-rich n value, below potarite which the.axis: The separates elemental number the box of values grains into twoin containing b partsare presented (i.e.,the lower element in 25th–50th atomic of prop interest. percentileortions The group (at.). colored andData polygon from upper 50th–75th percentile). The n below the axis: number of grains containing the element of interest. The colored poly- in bCabri limits [25] the have ﬁeld been for Au-richincluded potarite.in b to be Thecompared elemental with valuesdata from in bthis are study. presented in atomic proportions (at.). Data from gon in b limits the field for Au-rich potarite. The elemental values in b are presented in atomic proportions (at.). Data from Cabri [25] have been included in b to be compared with data from this study. Cabri [25] have been included in b to be compared with data from this study.

Figure 15. A selection of backscatter electron (BSE) images showing PGMs and Au association. (a) Figure 15. A selection of backscatter electron (BSE) images showing PGMs and Au association. (a) FigureAn Au-rich 15. A (46.65 selection wt%) potarite; of backscatter (b,c) Mertieite-I electron (M-I; (BSE) Pd11(Sb,As) images4) showingpartly encapsulated PGMs and in gold; Au association. An Au-rich (46.65 wt%) potarite; (b,c) Mertieite-I (M-I; Pd11(Sb,As)4) partly encapsulated in gold; (a)(d An) Isomertieite Au-rich (IsM; (46.65 Pd wt%)11Sb2As potarite;2) as an inclusion (b,c) Mertieite-I in a gold particle; (M-I; Pd(e) 11A (Sb,As)vincentite4) (Vin; partly encapsulated in (d) Isomertieite (IsM; Pd11Sb2As2) as an inclusion in a gold particle; (e) A vincentite (Vin; gold; (d) Isomertieite (IsM; Pd11Sb2As2) as an inclusion in a gold particle; (e) A vincentite (Vin; (Pd,Pt)3(As,Sb,Te)) attached to a gold grain; (f) Vysotskite (Vys; (Pd,Ni)S) enclosed by a gold aggregate; (g) An atheneite (Atn; (Pd,Hg)3As) attached to a gold grain with silicate inclusions; (h) Platinum (Pt) inclusion in a gold grain; and (i) Gold precipitation/nucleation on the surface of a sperrylite (Sp;

PtAs2). Minerals 2021, 11, x FOR PEER REVIEW 18 of 28

Minerals 2021, 11, 264 (Pd,Pt)3(As,Sb,Te)) attached to a gold grain; (f) Vysotskite (Vys; (Pd,Ni)S) enclosed by a gold ag- 18 of 28 gregate; (g) An atheneite (Atn; (Pd,Hg)3As) attached to a gold grain with silicate inclusions; (h) Platinum (Pt) inclusion in a gold grain; and (i) Gold precipitation/nucleation on the surface of a sperrylite (Sp; PtAs2).

Figure 16. A selection of backscatter electron (BSE) images of platinum-group minerals (PGMs) Figure 16. A selection of backscatter electron (BSE) images of platinum-group minerals (PGMs) forming polymineralic aggregates. Two-phase PGMs composed of (a) Sperrylite (Sp; PtAs2) and formingirarsite (Ira; polymineralic (Ir,Ru,Rh,Pt)AsS); aggregates. (b) Pt-rich Two-phase irarsite (Ira-Pt) PGMs and composed Ru-rich irarsite of (a )(Ira-Ru); Sperrylite (c) Ruthen- (Sp; PtAs2) and irarsiteiridosmine (Ira; (Rut; (Ir,Ru,Rh,Pt)AsS); (Ir,Os,Ru)) and (erlichmaniteb) Pt-rich irarsite (Erl; OsS (Ira-Pt)2); (d) Rutheniridosmine and Ru-rich irarsite and (Ira-Ru);platarsite ((Plt;c) Rutheniri- (Pt,Rh,Ru)AsS); (e) Platarsite and sperrylite; and (f) A three-phase PGM composed of sperrylite, dosmine (Rut; (Ir,Os,Ru)) and erlichmanite (Erl; OsS2); (d) Rutheniridosmine and platarsite (Plt; platarsite and hollingworthite (Ho; (Rh,Pt,Pd)AsS). (Pt,Rh,Ru)AsS); (e) Platarsite and sperrylite; and (f) A three-phase PGM composed of sperrylite, platarsite3.4. PGMs and Shape, hollingworthite Grain Size Distri (Ho;bution, (Rh,Pt,Pd)AsS). and Surface Texture in Till 3.4. PGMsMorphology Shape, Grainof PGM Size grains Distribution, is more andlikely Surface influenced Texture by in their Till initial crystalline growth than post-erosion processes. Unlike metallic gold, which is malleable and the shapeMorphology of which is expected of PGM to grains be modified is more du likelyring transport, influenced morphologies by their initial of placer crystalline PGMs growth thanare reported post-erosion to be diverse processes. and resistant Unlike to metallic mechanical gold, modification which is malleable [59]. While andgold the has shapea of whichVickers is hardness expected number to be (VHN) modified of 30 during to 60 (10 transport, g load), platinum morphologies or palladium of placerhas a VHN PGMs are reportedof 300 to 340 to be (100 diverse g load), and and resistantmost PGMs to have mechanical a VHN in modification the range of 500–3000 [59]. While (25 to gold100 has a Vickersg loads). hardness Consequently, number even (VHN) PGM of 30alloys to 60do (10 not g load),abrade platinum and preserve or palladium their original has a VHN ofshapes. 300 to 340Some (100 PGM g load), can be and brittle, most and PGMs mayhave fracture a VHN during in thethe rangeerosion of process. 500–3000 At (25the to 100 g loads).most, edges Consequently, may show evensome PGMevidence alloys of abrasion, do not abradethe cause and of which preserve might their but original not nec- shapes. Someessarily PGM be canrelated be brittle,to their andtransport. may fracture BSE images during in Figure the erosion 17 shows process. a wide At variety the most, of edges morphologies and surface textures of till-hosted sperrylite, including: preserved original may show some evidence of abrasion, the cause of which might but not necessarily be euhedral shape and angular edges (Figure 17a,d), semi-rounded edges (Figure 17e,f), related to their transport. BSE images in Figure 17 shows a wide variety of morphologies rounded edges (Figure 17g–i), breakage surfaces (Figure 17j–l), monomineralic aggregates andand/or surface twinned textures crystals of (Figure till-hosted 17l–o,r), sperrylite, intergranular including: textures preserved(Figure 17p,q) original that are euhedral possibly shape andimposed angular by neighboring edges (Figure minerals 17 a,d),during semi-rounded mineral growth, edgesa botryoidal (Figure form 17 (Figuree,f), rounded17r), and edges (Figuresurfaces 17 stainingg–i), breakage (Figure surfaces17c,e,s). In (Figure spite of 17 thisj–l), diversity, monomineralic as also illustrated aggregates in and/orprevious twinned crystalsfigures,(Figure the majority 17l–o,r), of PGMs intergranular retained their textures original (Figure shape, 17 whilstp,q) that mechanically are possibly induced imposed by neighboringdamages on mineralsgrains is limited during to mineral various growth,types of shallow a botryoidal and deep form fractures (Figure on 17 theirr), and sur- surfaces stainingfaces (e.g., (Figure linear 17 andc,e,s). conchoidal In spite fractures, of this diversity, straight asand also accurate illustrated grooves, in previouscrescentic figures, thegouges majority and troughs) of PGMs [49,60]. retained It is their uncertain original if these shape, damages whilst were mechanically induced by induced erosion of damages onthe grains host material, is limited or after to various their liberation types of from shallow the host and rock. deep Given fractures the lack on oftheir information surfaces (e.g., linearin the andliterature conchoidal regarding fractures, surface textures straight and and morphology accurate of grooves, <50 µm PGM crescentic grains in gouges till, and troughs)it is difficult [49, 60to]. correctly It is uncertain interpret ifthe these surfac damagese textures werein relation induced to crystallization by erosion ofand the host material, or after their liberation from the host rock. Given the lack of information in the literature regarding surface textures and morphology of <50 µm PGM grains in till, it is difficult to correctly interpret the surface textures in relation to crystallization and subsequent erosional transport. Larger grains can be subject to more mechanical damage during transport (colliding with other grains) given their larger surface area. Minerals 2021, 11, 264 19 of 28

Minerals 2021, 11, x FOR PEER REVIEW 20 of 28

Figure 17. A selection of backscatter electron (BSE) images showing various shapes and morpholo- Figuregies of 17.sperryliteA selection grains re ofcovered backscatter from till electron samples. ( (BSE)a–d) Pristine images grains. showing (e–f) Semi-rounded various shapes and morphologies ofgrains. sperrylite (g–i) Rounded grains grains. recovered (j–l) Grains from with till breakage samples. surfaces. (a–d) (m Pristine–o) Monomineralic grains. ( aggre-e–f) Semi-rounded grains. gates / twinned crystals. (p,q) Grains with intergranular textures. (r) Botryoidal form of a sperry- (glite.–i) Rounded(s) Grain with grains. corroded (j– lsurface.) Grains with breakage surfaces. (m–o) Monomineralic aggregates/twinned crystals. (p,q) Grains with intergranular textures. (r) Botryoidal form of a sperrylite. (s) Grain with corroded surface.

Figure 17 shows that the edges of some sperrylite grains are variably abraded, which might develop during transport [61,62]. Detrital grains transported by glaciers, especially at the base of thick ice sheets, are expected to show high relief induced by fracturing [60]. In contrast, low and medium relief can be the result of abrasion during long-distance transport, reworking, and/or transport in mixed environments (e.g., glaciofluvial environments) [60]. No significant difference in PGM abundance is noted between the lodgment till where the material is encapsulated in impermeable compacted sediments, compared to the ablation till or other type of glacial sediments where the material has been in protracted contact with melt or groundwater. This suggest that chemical corrosion of grains induced by oxidation is limited, comparatively to sulfides for instances. Understanding the relation between abrasion texture and aqueous sedimentation requires a systematic investigation, which was not part of current project. Nevertheless, the surface textures and morphology of PGMs documented in this study are contrasting from those from intensely corroded and abraded PGM grains from beach and aeolian environments (e.g., platinum beach placers, southern New Zealand) [63]. The range of grain sizes of different PGM species (n = 963) recovered from till samples collected in a single project from the James Bay territory is illustrated in Figure 18a, the Minerals 2021, 11, 264 20 of 28

vast majority of which being in the 10 to 20 µm range. No marked difference is noted between mineral species. This range is comparable with the grain size distribution of all sperrylite grains (n = 1488) found in all studied samples from various geological settings (Figure 18b). Although there are some examples of research on placer PGMs in the literature, e.g., [56], the grain size and shape distribution of PGMs transported by glaciers have not yet been comprehensively researched, considering the difficulty to recover them in the course of routine analysis. The grain size distribution of samples prepared for automated SEM-based analyses are bound by instrumental limitations such that grains smaller than approximately 10 µm are commonly not efficiently recovered during gravimetric concentration (the fluidized bed recovery shows a collapse at about 20 µm; [4]), whereas grains larger than 50 µm are excluded at final sieving. In Figure 18a,b, the presence of grains with lengths larger than 50 µm may be explained by passing through the sieve from their smaller surface area (width). Nevertheless, most grains are relatively stubby, and complex shapes are rare (Figure 18). It can be interpreted that various mineral species show similar grain size distribution except for sperrylite being statistically larger (Figure 18a) but it could be better expressed because they are statistically more abundant. As a result, there is no significant grain size difference in sperrylite recovered from various regions, and thus Minerals 2021, 11potential, x FOR PEER REVIEW source rocks. 21 of 28

Figure 18.Figure Grain size 18. distributionGrain size of distributionplatinum-group ofminerals platinum-group (PGMs) occurring minerals in the < (PGMs) 50 µm heavy occurring mineral insuper- the <50 µm concentrates from glacial till sediments. (a) Range of PGMs’ width vs. length measurements for the various mineral species recoveredheavy from samples mineral collected super-concentrates in the James Bay fromterritory, glacial Archean till Supe sediments.rior Craton (a (Quebec,) Range Canada). of PGMs’ (b) Range width of vs. length width vs. measurementslength measurements for of the sperrylit variouse grains mineral from various species geological recovered setting froms within samples the Canadian collected Shield. inMajority the James Bay of the grains show similar size distributions regardless of their mineral species and/or sampling area. territory, Archean Superior Craton (Quebec, Canada). (b) Range of width vs. length measurements of sperrylite grains4. Discussion from various geological settings within the Canadian Shield. Majority of the grains show similar size distributionsDetrital heavy regardlessindicator mineral of their grains mineral are globally species used and/or to explore sampling potential area. mineral deposits in up-stream/ice locations [8,33,64,65]. Previously, given poor efficiency of processing techniques, the PGMs could be rarely recovered from glacial sediments such that the presence of a couple of grains could indicate a nearby PGE mineralization [11,66]. Im- provements in methods for recovery and recognition of precious metal minerals in unconsolidated sediments accompanied by recent advancements in analytical techniques have led to this study presenting a large mineralogical and chemical database of silt-sized PGM grains recovered from till sampled for gold exploration and not as traditionally for Ni-Cu-PGE deposits. Although this study shows that sampling protocols developed for gold recovery from sediments and its geochemical characterization are also applicable to PGMs, as physicochemical characteristics of PGMs differ from gold, these protocols still need to be

Minerals 2021, 11, 264 21 of 28

4. Discussion Detrital heavy indicator mineral grains are globally used to explore potential mineral deposits in up-stream/ice locations [8,33,64,65]. Previously, given poor efficiency of processing techniques, the PGMs could be rarely recovered from glacial sediments such that the presence of a couple of grains could indicate a nearby PGE mineralization [11,66]. Improvements in methods for recovery and recognition of precious metal minerals in unconsolidated sediments accompanied by recent advancements in analytical techniques have led to this study presenting a large mineralogical and chemical database of silt-sized PGM grains recovered from till sampled for gold exploration and not as traditionally for Ni-Cu-PGE deposits. Although this study shows that sampling protocols developed for gold recovery from sediments and its geochemical characterization are also applicable to PGMs, as physicochemical characteristics of PGMs differ from gold, these protocols still need to be optimized for PGE exploration. For instance, PGMs hardness is higher than gold (2.5 in Mohs scale) contributing to their resistance to deformation and enhancing their fragility against mechanical forces. Consequently, the shape and morphology of PGMs will evolve differently from gold during transport, and therefore, the qualitative morphological classifications proposed for gold to estimate the distance of transport and proximity to the source [50] are not applicable in exploration for PGMs. Oberthür [67] investigated the behavior of PGMs in the exogenic environment and showed that different PGMs react differently to the weathering process such that Pt-bearing minerals, sperrylite in particular, show much higher rates of survival compared to Pd- bearing phases. However, this study indicates rare occurrence of Pd-bearing minerals as inclusions in other mineral phases or with corroded surfaces, which can suggest limited destruction of these in the glacial environment. Based on the literature, postglacial weathering may change chemical and mineralogical composition of till in oxidation zones (e.g., decomposition of sulfides) causing depletion or enrichment of metal contents in soil in comparison to un-weathered till [68–70]. The given phenomenon has not been recorded in this study since sampling of oxidized material have been avoided, and C-horizon till supposedly provides an unimpeded signal of clastic glacial dispersal that is least affected by weathering. Therefore, the result of processing of 5194 till samples shows a similar frequency of occurrence of PGMs in < 50 µm HMC from different types of till samples, regardless to their contact with oxidizing ground water.

4.1. Origin of Platinum-Group Minerals in Till Discussing the metallogenic significance of 58 different PGM species found in till is difficult, especially considering that these minerals are free grains detached from their source rocks, and the mineral composition of the sediments has not been assessed. No efforts have been made yet as to locate the source of these PGMs in the studied areas. Thus, concluding that they must originate from some ultramafic rocks is not supported by any evidence yet. Sperrylite is by far the most common PGM recovered in this study, as well as been reported as abundant in many other studies [8,71,72]. However, sperrylite abundance will not differentiate between PGE-bearing deposit types as it is observed in a great variety of settings, from reef-type to PGE-rich base metal sulfide occurrences, to layered intrusions chromitites through ophiolitic chromitites [73]. The same limitations apply to the cooperite-braggite series regarding geological settings. As these two groups of minerals (sperrylite and cooperite-braggite) represent almost 75% of the entire dataset obtained from the till samples, it is difficult to interpret the sources of these minerals. The platinum-group minerals belonging to the arseno-antimonide series isomertieite- mertieite represent altogether 6% of the PGM population recovered in this study. Like the aforementioned minerals, the presence of mertieite in till is not also discriminant for any specific deposit type [73]. Minerals from the laurite-erlichmanite series and the IPGE alloys (4.2%) can possibly be used as diagnostic minerals. It is true that these minerals are Minerals 2021, 11, 264 22 of 28

generally associated with ultramafic source rocks in general, and to chromite-rich rocks in particular, these settings are not exclusive. Figures 19 and 20 show the distribution of the most abundant identified PPGE PGMs in the studied samples including sperrylite, braggite, cooperite, isomertieite, mertieite-I, and mertieite-II in the (Pt at. + Pd at. + Rh at.) vs. (S at. + As at.) biplot, and IPGE PGMs including anduoite, erlichmanite, laurite, and irarsite in the (Ru at. + Os at. + Ir at.) vs. (S at. + As at.) biplot. As a result, PPGE PGM species from different geological settings show almost similar range of compositions of the analyzed elements (Figure 19). In contrast, the distribution of IPGE PGMs is mainly limited to the samples collected from the Superior Province (Figure 20). As mentioned earlier, grains < 50 µm face limitations imposed by the beam size for analysis by LA-ICP-MS in order to characterize their trace element signature. Hence, given that samples from the Superior Province come from various sub-provinces including La Grande, Eastern Abitibi, Western Abitibi, Nemiscau, Opinaca, Assinica Greenstone belt, Wawa Greenstone belt, Quetico Greenstone belt, and the Otish basin, mineral-chemical characterization of PGMs in potential host rocks is required to reveal whether the similar chemistry of the grains indicates the same type of mineralization or simply demonstrates incapability of major and minor elements in provenance discrimination of detrital PGMs. Nevertheless, the ubiquity of PGM and the paucity of ultramafic rocks in the Superior Minerals 2021, 11, x FOR PEER REVIEWProvince suggest that other source types may be involved. The dominance of PPGE23 PGMs of 28

over IPGE PGMs, in which the latter are demonstrated as preferentially derived from chromitites [74], further supports this hypothesis. This suggests that a proportion of PGMs Assinicaoccurring Greenstone in the till are belt, rather Wawa related Greenstone to non-ultramaﬁc belt, Quetico sources, Greenstone such asbelt, to hydrothermaland the Otish basin,events mineral-chemical rather than being concentratedcharacterization by early,of PGMs high-temperature, in potential host magmatic rocks processesis required [73 to]. revealThis is whether supported the by similar the occurrence chemistry of minorof the amountgrains indicates of platinum the andsame palladium type of mineraliza- as alloying tionmetal orin simply gold grainsdemonstrates and which incapability could have of major been transported,and minor elements as gold, in by provenance hydrothermal dis- criminationmineralization of detrital events ([PGMs.73] and references therein).

Figure 19. Covariation of (Pt at. + Pd Pd at. at. + + Rh Rh at.) at.) vs. vs. (As (As at. at. + +S Sat.) at.) in in the the composition composition of of relatively relatively abundant platinum-group minerals (PGMs) including sperrylite, braggite, cooperite, isomertieite, abundant platinum-group minerals (PGMs) including sperrylite, braggite, cooperite, isomertieite, mertieite-I, and mertieite-II recovered from till collected from different geological settings within mertieite-I, and mertieite-II recovered from till collected from different geological settings within the the Canadian Shield. The elemental values are presented in atomic proportions (at.). Although regionalCanadian differences Shield. The are elemental noted, most values of thes are presentede minerals inshow atomic ubiquitous proportions distributions. (at.). Although regional differences are noted, most of these minerals show ubiquitous distributions.

Figure 20. Covariation of (Ru at. + Os at. + Ir at.) vs. (As at. + S at.) in the composition of relatively abundant iridium-like platinum-group elements (IPGE)-rich platinum-group minerals (PGMs) including anduoite, erlichmanite, laurite, and irarsite recovered from till collected from different geological settings within the Canadian Shield. The elemental values are presented in atomic proportions (at.). Although regional differences are noted, most of these minerals show ubiquitous distributions.

Minerals 2021, 11, x FOR PEER REVIEW 23 of 28

Assinica Greenstone belt, Wawa Greenstone belt, Quetico Greenstone belt, and the Otish basin, mineral-chemical characterization of PGMs in potential host rocks is required to reveal whether the similar chemistry of the grains indicates the same type of mineralization or simply demonstrates incapability of major and minor elements in provenance discrimination of detrital PGMs.

Figure 19. Covariation of (Pt at. + Pd at. + Rh at.) vs. (As at. + S at.) in the composition of relatively abundant platinum-group minerals (PGMs) including sperrylite, braggite, cooperite, isomertieite, Minerals 2021, 11, 264 23 of 28 mertieite-I, and mertieite-II recovered from till collected from different geological settings within the Canadian Shield. The elemental values are presented in atomic proportions (at.). Although regional differences are noted, most of these minerals show ubiquitous distributions.

FigureFigure 20.20. Covariation of of (Ru (Ru at. at. + Os + Osat. at.+ Ir +at.) Ir vs. at.) (A vs.s at. (As+ S at.) at. in + the S at.) composition in the composition of relatively of relativelyabundant abundantiridium-like iridium-like platinum-group platinum-group elements (IPGE)-rich elements (IPGE)-rich platinum-group platinum-group minerals (PGMs) minerals (PGMs)including including anduoite, anduoite, erlichmanite, erlichmanite, laurite, and laurite, irarsite and recovered irarsite recovered from till collected from till from collected different from differentgeological geological settings within settings the within Canadian the CanadianShield. The Shield. elemental The valu elementales are presented values are in presented atomic pro- in portions (at.). Although regional differences are noted, most of these minerals show ubiquitous atomic proportions (at.). Although regional differences are noted, most of these minerals show distributions. ubiquitous distributions.

Inversely, this study establishes a common relation between PGM and gold grains in the till, suggesting that PGMs might be used as indicator minerals for gold, and sim-

ply added to automated SEM analyses done for gold. Sites where till samples contain gold grains enriched in Hg also contain potarite (PdHg), Au-rich potarite, and palladian- weishanite (Pd ± PtAuHg). In addition, the presence of PGMs attached to, or partly included in, gold particles and the presence of Au-Ag-bearing PGMs and Au-rich PGMs in the till samples clearly indicates an association of PGE and gold. Although Au-PGE mineralization is poorly documented in the existing literature, the apparently atypical PGM population shown in this study could be commonly associated with gold mineralization [75].

4.2. Suggestions for PGM Exploration The methodology to detect rare PGM in glacial sediments from eroded source rocks used in this study works equally well for barren/mineralized terrains with respect to PGE. Till mineralogy can thus be used as exploration tools to target PGE-enriched sources such as reef-type, PGE-rich Ni-Cu sulfide deposits or PGE-rich chromitites, without the need to rely upon proxies such as magnesian silicates, chromian spinels, or nickel sulfides. Since mineral-based methods enable far lower detection limits than chemical analysis, the method is expected to be far more sensitive than any assay-based method. The difficulty in tracking a particular type of deposit comes from the diversity of PGM species found in each deposit types, even at the sample scale [76–78]. Another difficulty arises from the diversity of PGM assemblages that seems to reflect the host lithologies more than the deposit type (the PGM assemblage of the silicate-rich facies differs from the PGM assemblage of the chromite-rich facies within the Merensky Reef, wherein variations occur within meters) [77]. Furthermore, the level of complexity is increased considering the morphological and chemical variations induced on these minerals in the secondary environment, variations that will differ from mineral species to species. The benefit of this method is that PGE-bearing minerals are recovered alongside with gold grains, without necessitating the use of a specific analytical method, and can thus be included in integrated surveys. Minerals 2021, 11, 264 24 of 28

It is clear that the analytical protocol presented here allows its use for the exploration of PGE-rich mineralization. Although, to clearly beneﬁt of its use, particular case studies should be performed: (i) to compare the PGM assemblage from the deposit source rocks with the PGM population recovered from the till samples, (ii) to compare the PGMs abundance between the source rocks and the till samples, (iii) to verify if the PGMs are texturally and chemically modiﬁed in a secondary environments, and (iv) to specify an appropriate sampling pattern and sample size for PGM density and dispersion, appropriate for PGE-rich mineralization.

5. Conclusions This study is the first one presenting a large mineralogical and chemical database of platinum-group minerals (n = 2664) recovered from < 50 µm heavy mineral fraction of till samples. It also presents a comprehensive methodology for pre-concentration and efficient recovery of precious metal minerals (PMM) from sediment samples, and preparing suitable sub-samples for characterization by automated SEM-based analytical techniques. The BSE images demonstrating shape, morphology, and surface textures of PGMs as well as geochemical data reported in this study provide a basis for future automated mineralogy case studies of till for exploration for PGE-rich resources in glaciated terrain of Canada and elsewhere. This study’s results can also be used for comparison with geochemical characteristics of PGMs in host magmatic and/or hydrothermal ore systems in order to establish mineral-chemical exploration models. Moreover, the major findings of this study include: 1. Fifty-eight different PGM species have been identified. Sperrylite is the most abundant PGM (55.86%; n = 1488;) followed by cooperite (9.01%; n = 240), braggite (8.52%; n = 227), Pt-dominated minerals (3.64%; n = 97), mertieite-II (2.74%; n = 73), laurite- erlichmanite (2.70%; n = 72), potarite (2.14%; n = 57), mertieite-I (1.73%; n = 46), isomertieite (1.58%; n = 42), Os ± Ir ± Ru-dominated alloys (1.54%; n = 41), and irarsite (1.28%; n = 34). 2. Our observations indicate an equal chance of PGMs occurrence in different types of till, including basal, melt-out, and reworked. This may suggest lower rates of chemical alteration in glaciated environments resulting in stability and better preservation of different PGM species. This hypothesis can be supported by mainly intact surfaces of recovered PGMs. 3. Gold-rich potarite grains and the occurrence of PGMs e.g., mertieite-I, isomertieite, vincentite, and vysotskite in gold-dominated aggregates likely suggest crystallization of at least part of recovered PGMs in Au-rich systems.

Author Contributions: Conceptualization, R.G., P.P. and S.M.; validation, S.M. and P.P.; formal analysis, S.M., P.P. and J.T.; investigation, S.M. and P.P.; writing, S.M., P.P., R.G. and J.T.; project administration, J.T. and R.G.; funding acquisition, R.G. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Data Availability Statement: Data available on request due to restrictions. The data presented in this study are available on request from the corresponding author. The data are not publicly available because these projects are not disclosed in order to preserve the rights of the clients of IOS Services Géoscientiﬁques. Acknowledgments: N. Fournier (IOS, laboratory manager), P. Villeneuve (IOS, quaternary geologist), A. Neron (IOS, research scientist), H. Longuepée (IOS-Senior scientist), K. Gagné (IOS-quality control) and M.O. Chartier (IOS, data management) for their technical support. D.H.C. Wilton (MUN) and G. Thompson (CNA) are also thanked for invitation to contribute to this special issue and for editorial handling. Anonymous reviewers are thanked for their constructive comments that improved the manuscript considerably. Minerals 2021, 11, 264 25 of 28

Conﬂicts of Interest: The authors being professional scientists or general manager (Girard) with IOS Services Géoscientiﬁques Inc., which is a commercially operated laboratory. They participated in the development of the analytical procedure assessed in this study and were involved in the design of the study, the analysis, and interpretation of data, and in the writing of the manuscript.

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